The embryotoxic, cytogenetic and carcinogenic effects of new as well as clinically useful antitumor agents are under investigation. The embryotoxicity of two adenosine deaminase inhibitors, 2'-deoxycoformycin and EHNA, alone and in combination with various adenosine analogs, is under investigation in mice, and studies on the embryotoxic and cytogenetic effects of two inhibitors of de novo pyrimidine biosynthesis (PALA and 6-azauridine) are in progress. The embryotoxic effects of anthracenedione and dihydroxyanthracenedione are also being evaluated in mice. The carcinogenic activity of various clinically effective antitumor and/or immunosuppressive agents, including procarbazine, N-methylnitrosourea, melphalan, azathiporine and adriamycin, is under evaluation in long-term studies in monkeys. A survey of second malignancies developing in treated cancer patients, initiated several years ago, is continuously being updated using data from published reports and from several centers and hospitals.